The present invention relates to printers and more specifically to printers which employ a thermal line printing head to print images on thermal sensitive paper and still more particularly to printers in which the print head is used in conjunction with a platen roll which is mounted in a label cassette which is removable from the printer.
Compact printers using thermal line print heads are well known and widely used. One application in which thermal printers have been used is in printing labels for packaged food items. It is not unusual to use such printers in the deli, fresh produce and meat departments of a grocery store to print labels. In a typical printer the print head is used in association with a print or platen roller. The print head typically abuts the platen roller during the printing operation and the platen roller holds the label stock against the face of the print head. The platen roll is frequently formed from rubber or a similar compound of sufficiently low durometer to provide a large area of contact. In many cases the platen roller also functions as a drive roll to advance the label stock through the printer. If the print head is not correctly positioned or becomes misaligned with the platen roller, the print quality can become very poor. In particular, image sharpness and intensity can diminish. Accordingly, in designing printers, it is generally necessary to provide some means for adjusting the alignment between the print head and the platen roll so as to enable the user to maintain good print quality.
Printers that are used in grocery stores in some cases have been designed with a removable cassette in which the label stock is supplied. It is not unusual to use multiple cassettes in conjunction with a single printer. For example, one cassette may carry labels of a first length and another cassette may be used to carry labels of a second length. Different cassettes may also be used to supply linered label stock and linerless label stock.
The use of interchangeable cassettes with a single printer presents problems in aligning the print head when the platen or print roller is housed within the cassette because the position of the platen roller relative to the print head may not be the same in one cassette as another and print quality may suffer. The difference in positions may be the result of manufacturing variances or the cassette not seating itself in the identical position each time it is loaded into the printer. Nevertheless, it is considered advantageous to mount the platen roller in the cassette because it provides easier access to the print head for cleaning and replacement and it also simplifies loading the label stock in the printer.
U.S. Pat. No. 5,694,159 to Kajuja et al discloses a thermal printer having a removable print head wherein the print head is biased by springs into contact with a platen roller. However, while this construction may be helpful in positioning the print head on the platen roller, this construction does not address the lateral adjustment that is often required when loading different cassettes into a single printer.